Chemical Defect
by imagia-quill
Summary: When one finds love, a certain part of their brain, tasked to translate their monochromatic sight into colors, is activated. But to Sherlock, sentiment is a chemical defect. John is determined to change this, but it's not as easy as he thought when he realized that the roots had traced back to Sherlock's dark childhood. Johnlock. S4-compliant, soulmate AU.


**Disclaimer**: Not Moffat, Gatiss, or Thompson. Definitely not SACD. Soulmate AU where you will see colours the moment you realize you've found love. S4 compliant, with mentions of Eurus Holmes and Victor Trevor, but does not take place in S4. Enjoy!

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**Chemical Defect  
**by: imagia-quill

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_Give me one night to mend the pain,  
because the second we touched you'll forget the day  
you were done with love."_

–Zedd, Done With Love.

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_Musgrave Hall, 1989_

Mycroft didn't see the world in colours, but he didn't mind.

The young genius, a teenage boy of thirteen years old, had read enough on the human nervous system to know that there was a special region of the brain called _griseothalamus _**[1]** that was activated to aid monochrome-sighted people to tell colours apart. The region translated the light signals from the retina into a more specific map of shades of grey to enable monochrome-sighted people to tell colours just as accurately. He could still tell the difference between the pinkish shade of his favorite strawberry jam and the specific lime green tint of the grasses growing on the meadow next to Musgrave Hall, although, if one would so poetically put it, he still couldn't fully comprehend the subjective term of beauty the chromatic-sighted people described about telling colors apart.

He was fine with it though, just as he was fine being a left-handed person in a world full of right-handed people. As long it didn't interfere with his studies, which sometimes required visual observations, he didn't really concern himself with such trivia.

Eurus, as fluent as she was at telling the slightest difference between two different shades, he knew didn't see colours either. At the early age of five, she could sort papers dripped with her own blood according to the length of time it had been exposed to air and oxidized just as easily as he could sort his ties by colours, not that Mycroft didn't shudder the first time she told him that. Nevertheless, she didn't show the particular appreciation towards colours the way Sherlock, his little brother, was completely awestruck the first time their mother showed them a prism or the way he really didn't move when he first saw a chameleon that one time their father took them to the local zoo.

Sherlock—it was clear for everyone in Musgrave Hall to see that Sherlock had seen colours.

His mother and he weren't really as convinced the first time his father told them Sherlock could see colours—they both had already read a lot of journals to know that humans rarely experienced colours from such an early age. The _amygdalochroma_ **[2]**, another region of the brain that translated the shades of gray into colours, were not usually to be mature until puberty, when the subject could finally start to understand the depth and the complexity of love and acceptance and respect and sacrifice and all the other things Mycroft really wasn't interested in.

But when Sherlock started calling himself Yellowbeard and Victor Trevor, the one person he seemed to be completely inseparable from, Redbeard, Mycroft knew who made his little brother see colours.

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One day Victor went missing.

Exactly one month after it, Sherlock, a seven years old curly-haired boy that could now stand observantly a good ten feet away from the chameleon cage in the local zoo, and seemed to develop an ability to delete not just the fact that he had a large burn mark snaking from his shoulder to his left upper arm that he received when Musgrave Hall had been burned down, but also the entire existence of his sister altogether.

It didn't take Mycroft long to figure out the actual extent of what had happened to Sherlock's sight and what he had done to his memory when he started talking about an Irish Setter named Redbeard.

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**[1]**_: Griseothalamus _(n.) from griseo- (from Latin _griseus, _"grey") + -thalamus. Thalamus is the largest subdivision of the diencephalon that serves chiefly to relay impulses and especially sensory impulses to and from the cerebral cortex.

**[2]**_: Amygdalochroma _(n.) from amygdalo- + -chroma (from Ancient Greek _khrôma, _"colour"). The amygdala is located medially within the temporal lobes of the brain in complex vertebrates, such as humans, and is shown in research to perform a primary role in the processing of memory, decision-making, and emotional responses.

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**A/N**: Whoop, okay, that's the first chapter! So, what do you think? Of course, the _griseothalamus _and _amygdalochroma _are both made up, but you know me, can't resist to mash words up and play with etymologies, hahahah! (To those of you who actually study human neurology, I'm really sorry if I don't get the references right! :( I don't study a lot of eukaryote biology.) Also, this is actually just a little idea that passed through my mind and wouldn't leave me alone, so I guess I should just write it. Do you think I should continue it? Constructive criticism is always welcome!


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